Next to the Beaufort Scale for the force of the wind, with a range from zero
to twelve, there is a similar scale describing sea conditions, with
categories from zero to nine.

The height of a wave is the vertical distance from
the top of a wave to the following trough.
The length of a wave
is measured from one wave crest to the next, or between two successive troughs.
The wave period is the observed time difference
between two successive crests.

The state of the sea consists not just of single waves, but of groups of
many waves, so-called ‘wave packets’. Such groups have five to seven
noticeably higher waves. A popular expression speaks of ‘the seventh wave’,
the one with the highest crest.